The present invention is generally related to chemical separation methods and, more particularly, is related to methods of separating thorium and plutonium.
The separation of thorium from plutonium has heretofore been accomplished by several methods, none of which has been altogether satisfactory.
In one process, plutonium has been separated from thorium by the method of solvent extraction, a method which is based on small differences in the respective extraction coefficients of plutonium and thorium. Multistage equipment including pulsed columns, which are expensive and complex, has been required for this process.
It has also been known to separate plutonium from thorium by a method based on anion exchange in nitric acid solutions. This method is based on small differences in distribution coefficients of plutonium and thorium and the resulting slow displacement of thorium by plutonium on the ion exchange resin. Clean-cut separation is seldom obtained because the exchange is slow and there is resulting mixing of the thorium and plutonium at the solution boundary.
It has also been known to separate plutonium from thorium by oxidation in aqueous solution of plutonium to Pu.sup.+6, followed by precipitation and separation of the thorium as a fluoride salt, and with subsequent precipitation of the plutonium as plutonium oxalate. However, satisfactory separation by this process has not been achieved on a plant scale for several reasons, most notably because of the difficulty in maintaining the valence of the plutonium in the +6 state. Failure to maintain the plutonium valence in the +6 state results in partial precipitation of the plutonium with the thorium. Also, complete precipitation of the thorium fluoride is difficult to obtain routinely, resulting in contamination of the plutonium with thorium.
Another method that has been previously known, and which is most closely related to the method of the present invention, is based upon chloride anion exchange from concentrated hydrochloric acid (HCl) solution. This method gives good separation but produces voluminous corrosive HCl fumes, thus requiring off-gas scrubbing to prevent corrosion of the various pieces of equipment and fume hoods which are normally used in the handling of plutonium.